1. Field of the Invention
The present invention relates to a method and apparatus for reproducing a system clock in a digital data transmission network.
There has been a rapid spread in transmission networks handling multimedia digital data in recent years. Such transmission networks normally are comprised of a plurality of nodes for sending or receiving or both sending and receiving digital data and transmission lines comprised of for example optical fibers for sending digital data among the nodes.
Source information to be sent from one node is coded at that node and sent as digital data through the transmission line to another node. The destination node decodes the received digital data to reproduce the original source information. In this process of codingxe2x86x92transmissionxe2x86x92decodingxe2x86x92reproduction, the coding is performed in synchronization with a system clock unique to the origination node of the source information, the transmission is performed by a frequency unique to the transmission line, and the decoding and reproduction are performed in synchronization with a system clock unique to the destination node of the source information. In a conventional transmission network, it was possible to send the source information by performing the above coding and decoding and reproduction using just the above system clock as the time base.
In a transmission network handling the above multimedia digital information, however, the source information may be diverse, that is, video information or audio information or may be computer information, so it is not enough to use just the system clock as the time base. In recent years, a time reference clock has been further incorporated to form the time base. This is so as to enable the destination node to reproduce the information in the packets (cells) in the order in which the origination node intended even if video packets (cells) containing video information and audio packets (cells) containing audio information are transmitted mixed with each other to the destination node.
Therefore, in the above explained transmission network handling multimedia digital information, the system clock at the destination node is reproduced in synchronization with the time reference clock contained in the digital data received from the origination node. The present invention relates to the reproduction of such a system clock synchronized with the time reference clock.
2. Description of the Related Art
As will be explained later with reference to the drawings, when expanding a point-to-point connection configuration transmission network (FIG. 19) into a point-to-multipoint connection configuration transmission network (FIG. 20 or FIG. 21), a means is required for switching channels.
If such a means for channel switching is interposed, however, the operation of switching the channel from a first node to a second node becomes necessary.
If a channel is switched from the first node to the second node, a monitor node which had reproduced its own system clock in synchronization with a first time reference clock from the first node, then, has to reproduce its own system clock in synchronization with a second time reference clock from the second node.
Since the first system clock in the first node and the second system clock in the second node are generated independently in each node, naturally the first time reference clock and the second time reference clock become asynchronous with each other. Therefore, the monitor node which had operated by a system clock synchronized with the first time reference clock becomes out of sync with the newly received second time reference clock immediately after switching the channel from the first node to the second node.
The out-of-sync monitor node then immediately starts a repull-in operation to synchronize its own system clock with the new second time reference clock. Due to this repull-in operation, however, the system clock in the monitor node ends up fluctuating.
When such fluctuation occurs in the system clock, a color irregularity appears in the image reproduced on the monitor of the monitor node and the image ends up being degraded in quality for a certain time. This degradation of the quality of the image lasts for several seconds. In the actual operation of a transmission network, however, when used successively switching images at the monitor in for example 5-second intervals such as from the camera of the first node to the camera of the second node and on to the camera of a third node, the problem arises that a normal image cannot be obtained on the monitor much at all. This poses a serious problem in the case of for example a multi-location television conference system where the participants in the conference are successively shown on a monitor by the cameras at those locations.
Therefore, an object of the present invention is to provide a method and apparatus for reproducing a system clock in a digital data transmission network which completes a repull-in operation of the system clock at the time of channel switching in an extremely short time and thereby prevents fluctuations in the system clock from occurring.
To attain the above object, according to the present invention, there is provided an apparatus for reproducing a system clock provided with a reproducing unit (21) for reproducing a first time reference clock (T1), a generating unit (26) for generating a system clock (Sck), a generating unit (22) for generating a second time reference clock (T2), a synchronization control unit (25) for minimizing based on the clocks (T1 and T2) the deviation between these clocks, a calculating means (23) for calculating a difference between counts of the first time reference clock (T1) counted in a predetermined time interval, and a calculating means (24) for calculating a difference between counts of the second time reference clock (T2) counted in a predetermined time interval. Thereby, the above deviation between the clocks can be minimized by inputting the above two calculated differences to the synchronization control unit (25). Thus, when reproducing digital data from a plurality of sending side nodes at a receiving node, high quality data can be reproduced even if switching channels from one node to another node.